Liquid pump using elastomeric piston, piston assembly and manufacturing method thereof

ABSTRACT

The invention provides a liquid pump using an elastomeric piston. The liquid pump has a piston assembly ( 10 ), which comprises: a piston rod ( 11 ); a piston head ( 12 ); a piston engaging portion ( 15 ) which is between the piston rod and the piston head in a necking shape; and a piston ( 13 ) made of an elastomeric material and sleeved around the piston engaging portion between the piston rod and piston head. The piston rod and piston head are formed separately and then assembled together. The piston engaging portion ( 15 ) is composed of a piston-engaging-portion formation part of at least one of the piston rod and the piston head. The invention also provides a method of manufacturing the piston assembly ( 10 ). The invention further provides a piston assembly ( 10 ) for a liquid pump. The piston assembly has a piston ( 13 ) which is provided with an annular notch ( 133 ) formed on the periphery of an upper end of the piston ( 13 ) where an annular inner flange ( 131 ) is formed. The piston assembly and its manufacture method are helpful to improving product precision and quality and facilitate automated assembly, and are specially used for the liquid pumps using an elastomeric piston.

FIELD OF THE INVENTION

This invention relates generally to a liquid pump, and particularly to a liquid pump which incorporates an elastomeric piston.

BACKGROUND OF THE INVENTION

A liquid pump is generally mounted at an opening of a container filled with an oil-like liquid, and used for dispensing the liquid out of the container by its user's manual press operation. Liquid pumps of this type commonly incorporate a piston made of an elastomeric material (such as Nitrile Butadiene Rubber (NBR)) but not of a plastic material, because the elastomeric materials are less susceptible to the oil-like liquids than the plastic materials which may generate deformation under the action of the oil-like liquids.

At present, there are various liquid pumps of the above type that incorporate an elastomeric piston and are specially used for dispensing oil-like liquid products. However, conventional liquid pumps of this type mostly have problems such as difficulty to manufacture and assembly and difficulty to improve the product precision.

A liquid pump of the above type is disclosed in JP2004-131163. As shown in FIGS. 1 a-1 e, the liquid pump comprises mainly a press head 120, a cylinder 130, an integrated threaded sleeve-cylinder cap 140, a spring 150, a lower non-return ball valve 160 and a piston assembly 110. The liquid pump is mounted at an opening of a container of liquid product, which opening is commonly an open top end of the container, and is used for dispensing the liquid product out of the container. The piston assembly 110 is moved up and down in the cylinder 130 in a well-known manner under the drive of the press head 120, thereby a desired pumping effect takes place. The piston assembly 110 includes a piston rod 111 and a piston head 112 which are integrally formed to be a one-piece longitudinally extending member, and a piston 113 which is fitted over a piston engaging portion 115 between the piston rod 111 and piston head 112. The piston engaging portion 115 is made to be a cylindrical necking portion between the piston rod 111 and piston head 112, and the cylindrical outer profile thereof mates with the cylindrical inner profile of the piston 113 which is fitted over the piston engaging portion. In the center of the integrated piston rod 111-piston head 112 there is formed a fluid passage 114, whose lower end is closed at the piston head 112 and whose upper end is open at the top end of the piston rod 111. At the piston engaging portion 115 there is formed a transverse passage 116, which extends radially through the piston engaging portion 115 substantially perpendicular to the fluid passage 114 and communicates with the fluid passage 114. The passage 116 can be regarded as an extension of the fluid passage 114, and the liquid in the liquid reservoir of the cylinder 130 can be pumped out of the container through the passages 114 and 116. The piston rod 111, at its joint with the piston engaging portion 115, has a conical step surface 1111 tapering towards the piston engaging portion 115, while the piston head 112, at its joint with the piston engaging portion 115, has a steep step surface 1121 which is substantially perpendicular to the longitudinal direction. The piston 113 is substantially in the form of a cylindrical sleeve, and is formed with an annular inner flange 1131 on its inside wall. The annular inner flange 1131 defines a round central hole, which is for the passage of the piston engaging portion 115 of the integrated piston rod 111 and piston head 112 and substantially surrounds the piston engaging portion 115. Opposite ends of the piston 113 are flared so as to form a lower sealing lip 1132 and an upper sealing lip 1133.

In the static assembled status of the liquid pump, the piston assembly 110 is slidably contained within the cylinder 130, with the annular inner flange 1131 of the piston 113 that is fitted over the piston engaging portion 115 closing off the transverse passage 116, and both the lower sealing lip 1132 and the upper sealing lip 1133 of the piston 113 abutting against the inside wall of cylinder 130. In this status, the fluid communication between the fluid passage 114 and the liquid reservoir of the cylinder 130 through the transverse passage 116 is blocked up by the annular inner flange 1131 of the piston 113, and therefore, the liquid in the liquid reservoir of the cylinder 130 cannot be dispensed out through the fluid passage 114.

As the press head 120 is pressed downwards against the force of the spring 150 and the piston assembly 110 is driven to move down therewith relative to the cylinder 130, both the sealing lips 1132 and 1133 of the piston 113 slide downwards while abutting against the sidewall of the cylinder 130. Owing to a combined action of the downward driving force exerted on the annular inner flange 1131 by the piston rod 111 and the piston engaging portion 115 and the upward friction force exerted on the sealing lips 1132 and 1133 of the piston 113 by the inside wall of the cylinder 130, as well as owing to a deformation space provided for the piston 113 by the conical step surface 1111 of the piston rod 111, the piston 113 is forced to deform toward the side of the conical step surface 1111, i.e., toward the side of the piston rod 111, as shown in FIG. 1 b. The deformation causes the annular inner flange 1131 of the piston 113 to offset from the piston engaging portion 115 so that the transverse passage 116, which is originally closed by the piston 113, is opened, thereby allowing the liquid in the liquid reservoir of the cylinder 130 to be pumped out through the transverse passage 116 and the fluid passage 114 under the effect of the downward pumping of the piston assembly 110. And then, as the press head 120 is released from the downward pressing force, the piston assembly 110 moves upwards relative to the cylinder 130 under the action of the spring 150. At this time, because the steep step surface 1121 of the piston head 112 does not provide a space for the piston 113 to deform towards the piston head 112, the piston 113 cannot deform considerably to allow the annular inner flange 1131 of the piston 113 to offset from the piston engaging portion 115 to release the sealing of the transverse passage 116. To sum up, the piston assembly 110 is configured as follows: when the piston assembly 110 is maintained stationary with respect to the cylinder 130, the piston 113 remains in its engaging position with the piston engaging portion 115, thereby to close the transverse passage 116 and prevent the fluid communication of the liquid reservoir of the cylinder 130 with the outside of the container through the fluid passage 114; as the piston assembly 110 is driven to move downwards relative to the cylinder 130, the piston 113 deforms and offsets from its engaging position to open the transverse passage 116 in the piston engaging portion 115, allowing the liquid reservoir of the cylinder 130 to communicate with the outside of the container; as the piston assembly 110 is moved upwards relative to the cylinder 130, the piston 113 still remains in its engaging position so as to close the transverse passage 116 and to prevent the fluid communication of the liquid reservoir of the cylinder 130 with the outside of the container through the fluid passage 114. Thus, in the piston assembly 110, the piston 113 fitted on the piston engaging portion 115 plays the role of an upper non-return valve, which cooperates with the lower non-return ball valve 160 to allow for the liquid loading and discharging of the liquid reservoir of the cylinder 130.

Also, JP52-112107 discloses a liquid pump of the above type. As shown in FIGS. 2 a-2 e, the pump is substantially the same in basic configuration as that in JP2004-131163 discussed above, with the difference only lying in the structure of the piston 213. Accordingly, the members in FIGS. 2 a-2 e are given the reference numerals similar to those in FIGS. 1 a-1 e just with the initial number “1” replaced with “2”, therefore, the same numerals, except for the initial number, denote the like members. In particular, the pump of JP52-112107 mainly comprises a press head 220, a cylinder 230, an integrated threaded sleeve-cylinder cap 240, a spring 250, a lower non-return ball valve 160 and a piston assembly 210. The piston assembly 210 includes an integrally formed and longitudinally extending one-piece piston rod 211/piston head 212, and a piston 213 which is fitted over the piston engaging portion 215 between the piston rod 211 and piston head 212. The piston 213 has a different configuration from that of the piston 113 in JP2004-131163, and is generally composed of an upper portion and a lower portion, the upper portion 2135 substantially presenting a cylindrical shape, and the lower portion 2136 substantially presenting a shape of frustum. The upper portion 2135, at its top end, is formed with an annular inner flange 2137 which defines a round central hole. The hole is for the passage of the piston engaging portion 215 between the piston rod 211 and the piston head 212, and surrounds the piston engaging portion 215. The lower portion 2136, as it extends downwards, gradually flares and gradually becomes thinner so as to form a sealing element of the piston 213 in contact with the inside wall of the cylinder 230. The thickness of the upper portion 2135 is significantly greater than that of the lower portion 2136.

The pump disclosed in JP52-112107 is operated in a similar manner to that in JP2004-131163. The piston assembly 210 is configured as follows: when the piston assembly 210 is stationary with respect to the cylinder 230, the piston 213 remains in its engaging position with the piston engaging portion 215, thereby to close the transverse passage 216 and prevent the fluid communication of the liquid reservoir of the cylinder 230 with the outside of the container through the fluid passage 214; as the piston assembly 210 is moved downwards with respect to the cylinder 230, the piston 213 deforms and offsets from its engaging position to open the transverse passage 216 in the piston engaging portion 215, allowing the liquid reservoir of the cylinder 230 to communicate with the outside of the container through the fluid passage 214; as the piston assembly 210 is moved upwards with respect to the cylinder 230, the piston 213 still remains in its engaging position so as to close the transverse passage 216 and to prevent the fluid communication of the liquid reservoir of the cylinder 230 with the outside of the container through the fluid passage 214. In particular, as the press head 220 is pressed downwards so as to drive the piston assembly 210 to move downwards relative to the cylinder 230 against the force of the spring 250, the piston 213 is made slide downwards with the end edge of its lower portion 2136 abutting against the sidewall of the cylinder 230. Owing to a combined action of the downward driving force exerted on the annular inner flange 2137 by the piston rod 211 and the piston engaging portion 215 and the upward friction force exerted on the end edge of the lower piston 2136 by the inside wall of the cylinder 230, as well as owing to a deformation space provided for the piston 213 by the conical step surface 2111 of the piston rod 211, the piston 213 is forced to deform toward the side of the conical step surface 2111, i.e., toward the side of the piston rod 211, as shown in FIG. 2 b. The deformation causes the annular inner flange 2131 of the piston 213 to offset from the piston engaging portion 215 so that the transverse passage 216, which is originally closed by the piston 213, is opened, thereby allowing the liquid in the liquid reservoir of the cylinder 230 to be pumped out through the transverse passage 216 and the fluid passage 214 under the action of the downward pumping of the piston assembly 210. And then, as the press head 220 is released from the downward pressing force, the piston assembly 210 moves upwards relative to the cylinder 230 under the action of the spring 250. At this time, because the steep step surface 2121 of the piston head 212 does not provide a space for the piston 213 to deform towards the piston head 212, the piston 213 cannot deform considerably to allow the annular inner flange 2131 of the piston 213 to offset from the piston engaging portion 215 to release the sealing of the transverse passage 216.

In the above two pump configurations of the prior art, the piston rod, the piston head and the reduced piston engaging portion between the two formers are formed to be a one-piece member, with a transverse passage in the piston engaging portion, therefore, it is necessary to use a composite mould including half moulds and a core to manufacture such a one-piece piston rod/piston engaging portion/piston head member. Such a manufacture method, using a complicated composite mould and thus involving multiple procedures such as withdrawing cores and splitting moulds, not only increases the equipment costs but also limits improvement of the productivity. Moreover, it is well known that the manufacture using half moulds has shortcomings such as low productivity and poor precision, wherein there exist inevitable problems of mould misalignment and flashing production due to the parting surface, which adversely affects the dimension precision of products, and finally, may lead to leakage of a finished assembly.

Another problem with the pump configurations of the prior art lies in that: in the one-piece piston rod/piston engaging portion/piston head, the outer diameter of the piston engaging portion for engaging with the piston is smaller than those of both the piston rod and the piston head, and the former is located between the two latter, thus, during engaging the piston onto the piston engaging portion, it is necessary to forcibly broaden the central hole of the annular inner flange of the piston to a size enough for the piston rod to pass so as to fit the piston onto the piston engaging portion. Such a fitting way is time- and labor-consuming, and is not suitable for an automatic assembly line. Moreover, in the course that the piston rod or piston head with a big diameter is inserted through the small central hole of the annular inner flange of the piston, the inner circumference of the hole is prone to be damaged, thereby adversely affecting the operation life of the piston assembly and thus of the liquid pump.

In addition, in regard to the piston configuration disclosed in JP52-112107, when the press head is pressed and moved downwards, under the effect of the upward friction force of the inside wall of the cylinder and the liquid pressure in the cylinder on the lower portion 2136 of the piston 213, the piston 213 should per se rapidly deform upwards so as to open the transverse passage 216 which is closed by the annular inner flange 2137. However, the upper portion 2135 of the piston 213 is rather thick and thus is not easy to deform, which may result in that the transverse passage 216 cannot be opened or can be opened but not in time, thereby causing the pump to be unable to operate normally.

SUMMARY OF THE INVENTION

In view of the shortcomings of the liquid pumps with an elastomeric piston of the prior art, the main object of this invention is to provide a liquid pump using an elastomeric piston that has improved product precision and quality and facilitates automatic assembly.

Another object of the invention is to provide a manufacture method of a piston assembly for liquid pumps that facilitates automatic production and is advantageous to ensure product precision and quality.

Still another object of the invention is to provide a piston assembly for liquid pumps that has improved operation reliability.

In order to attain the above object, in accordance with one aspect of the present invention there is provided a liquid pump using an elastomeric piston, for mounting on a liquid container to pump liquid in said container out of said container, said liquid pump comprising a piston assembly which is disposed in a cylinder slidably in a longitudinal direction of the pump, said cylinder defining a liquid reservoir for accommodating liquid therein. Said piston assembly comprises: a piston rod, formed with a fluid passage in a center of the piston rod in the longitudinal direction; a piston head, connected to a lower end of said piston rod; a piston engaging portion in the form of a neck between said piston rod and said piston head, said piston head being connected with said piston rod via said piston engaging portion, said piston engaging portion being formed with a transverse passage that diametrically extending through said piston engaging portion and is in fluid communication with said fluid passage so as to provide a passage for the liquid accommodated in said liquid reservoir to be pumped out of said container; and a piston made of an elastomeric material, fitted over said piston engaging portion between said piston rod and said piston head for releasably closing said transverse passage. Said piston assembly is configured such that: when said piston assembly is stationary with respect to said cylinder, said piston remains in its engaging position with said piston engaging portion so as to close said transverse passage and block the fluid communication of said liquid reservoir with the outside of said container through said fluid passage; as said piston assembly is moved downwards relative to said cylinder, said piston deforms to offset from said engaging position so as to open said transverse passage in said piston engaging portion and allow said liquid reservoir to be communicate with the outside of said container through said fluid passage; as said piston assembly is moved upwards relative to said cylinder, said piston remains in said engaging position so as to close said transverse passage and block the fluid communication of said liquid reservoir with the outside of said container through said fluid passage. The liquid pump of the present invention is featured in that said piston rod and said piston head are formed separately and then assembled together, said piston engaging portion being composed of a piston-engaging-portion formation part of at least one of said piston rod and said piston head.

By the way, it is noted that the words “upper” and “lower” and the like used herein are taken with respect to the orientation in which a liquid pump is normally used (i.e., the liquid pump is oriented such that the longitudinal direction of the liquid pump coincides with the vertical direction).

With the liquid pump using an elastomeric piston of the present invention, as the piston rod and the piston head are separately provided and then assembled together, the problems of complicated manufacture process and low productivity that are associated with the production of the integrated piston rod-piston engaging portion-piston head needing the complicated composite mould can be avoided. Moreover, it is unnecessary for the invention to use half moulds, so the invention avoids the problems of low productivity and poor precision which are usually associated with the employment of half moulds.

According to a preferred embodiment of the invention, said piston head comprises a base and a guide stub which extending upwards form said base, said piston rod being formed with a guide stub receiving hole at its lower end, said guide stub receiving hole being formed as part of said fluid passage, said guide stub portion being inserted into and fitted to said guide stub receiving hole so as to assemble said piston head onto said piston rod, said guide stub being formed with at least one longitudinal slot along a longitudinal length of said guide stub, said longitudinal slot constituting a longitudinal passage in said guide stub receiving hole, said piston engaging portion being formed with said transverse passage corresponding to said longitudinal slot in number, with said longitudinal slot correspondingly communicating with said transverse passage in said piston engaging portion, thereby providing a passage from said transverse passage to said fluid passage.

With such a configuration, it is very simple to fix the piston rod and piston head together by virtue of their own structures without needing any separate fixing means, which makes assembly process simpler and easily achieves the automatic production. Furthermore, once the piston rod and piston head are connected together, the necessary fluid passage, transverse passage and longitudinal passage will all be ready without needing any additional processing procedures, thus minimizing the production cost.

According to another preferred embodiment of the invention, said piston engaging portion is provided by both of said piston rod and said piston head, wherein said piston rod, at its lower end, is formed with a cylindrical reduced diameter tube portion which serves as one of said piston-engaging-portion formation parts, said reduced diameter tube portion defining part of said guide stub receiving hole, and wherein said piston head, at an upper end of its base, is formed with a cylindrical reduced diameter boss which serves as the other of said piston-engaging-portion formation parts, said reduced diameter tube portion and said reduced diameter boss having the same diameter, whereby said reduced diameter tube portion and said reduced diameter boss abut against each other to cooperatively constitute said piston engaging portion when said piston head is assembled to said piston rod.

According to the invention, said transverse passage of said piston engaging portion can be formed in said reduced diameter boss. Said transverse passage of said piston engaging portion can be formed integrally with said longitudinal slot of said guide stub. Of course, said transverse passage of said piston engaging portion can be formed in said reduced diameter tube portion, alternatively, can be formed in both said reduced diameter tube portion and said reduced diameter boss.

According to the invention, said piston engaging portion can be provided only by said piston head, wherein said piston head, at an upper end of its base, is formed with a cylindrical reduced diameter boss which serves as said piston-engaging-portion formation part, when said piston head is assembled to said piston rod, said reduced diameter boss abutting against a lower end of said piston rod to cooperatively constitute said piston engaging portion. Of course, said piston engaging portion can be provided only by said piston rod, wherein said piston rod, at its lower end, is formed with a cylindrical reduced diameter tube portion which serves as said piston-engaging-portion formation part, said reduced diameter tube portion defining part of said guide stub receiving hole, when said piston head is assembled to said piston rod, said reduced diameter tube portion abutting against an upper end of said base of said piston head to constitute said piston engaging portion.

According to the invention, said guide stub can be formed with a recess or protrusion along its periphery, and a mating protrusion or recess is formed along an inner circumferential surface of said guide stub receiving hole, helping to assemble said piston head to said piston rod.

According to the invention, in the case that the liquid pump comprises an integrated threaded sleeve-cylinder cap and a press head, said cylinder can be connected with said integrated threaded sleeve-cylinder cap on a lower side thereof, said press head being connected to an upper end of said piston rod, said press head having an upper stop position and a lower stop position of its press stroke with respect to said integrated threaded sleeve-cylinder cap, said integrated threaded sleeve-cylinder cap including an inner sleeve and an outer sleeve which extend in the longitudinal direction, said outer sleeve for engaging an open neck of said container so as to mount said liquid pump on said container, said inner sleeve for engaging an upper end of said cylinder so as to connect said cylinder to said integrated threaded sleeve-cylinder cap on the lower side thereof and thus to form a closed cylinder, when said press head is at the upper stop position of said press stroke with respect to said integrated threaded sleeve-cylinder cap, a lower end of said inner sleeve being engaged in said annular notch of said piston.

According to the invention, in the case that the cylinder is formed in its sidewall with a balancing vent for balancing pressures within and outside the container, when said press head is at the upper stop position of said press stroke with respect to said integrated threaded sleeve-cylinder cap, said balancing vent can be above an abutment point of said sealing lip of said piston with the sidewall of said cylinder, and below the abutment point of said inner sleeve of said integrated threaded sleeve-cylinder cap with said piston.

According to the invention, said piston rod, at its joint with said piston engaging portion, can have a conical step surface tapering towards said piston engaging portion, and said piston head, at its joint with said piston engaging portion, has a steep step surface which is substantially perpendicular to the longitudinal direction, said conical step surface providing a space for said piston to deform. Said piston engaging portion, at its joint with said conical step surface, can have an increased diameter portion.

Moreover, in accordance with another aspect of the present invention, there is provided a method of manufacturing said piston assembly of the above liquid pump, said method comprises: separately molding and providing said piston rod, said piston head and said piston; fitting said piston over one of said piston rod and said piston head that is suitable for receiving said piston; assembling said piston rod and said piston head together, with said piston being fitted around said piston engaging portion between said piston rod and said piston head.

The method of the invention does not need any complicated composite mould and does not involve the step of enlarging the piston's inner diameter which is necessitated in the traditional process, therefore, the piston assembly can be produced easily and at a lower cost, and with premium quality and higher reliability of the final product. In addition, the method facilitates the automatic production and thus achieves improved productivity.

In the method of the invention described above, in the case that each of the piston rod and the piston head has the piston-engaging-portion formation part, the piston can be received by the piston-engaging-portion formation part of either of the piston rod and the piston head, that is, both of the piston rod and the piston head are suitable for receiving the piston; in the case that the piston head has a guide stub for inserting into the guide stub receiving hole of the piston rod, it is preferred that the piston be received by the guide stub of the piston head.

Furthermore, in accordance with still another aspect of the present invention, there is provided a piston assembly for use in a liquid pump for mounting to a liquid container to pump liquid in said container out of said container, said liquid pump comprising a piston assembly which is disposed in a cylinder slidably in a longitudinal direction of the pump, said cylinder defining a liquid reservoir for accommodating liquid therein. Said piston assembly comprises: a piston rod, formed with a fluid passage in a center of the piston rod; a piston head, connected to a lower end of said piston rod; a piston engaging portion in the form of a neck between said piston rod and said piston head, said piston head being connected with said piston rod via said piston engaging portion, said piston engaging portion being formed with a transverse passage that diametrically extending through said piston engaging portion and is in fluid communication with said fluid passage so as to provide a passage for the liquid accommodated in said liquid reservoir to be pumped out of said container; and a piston made of an elastomeric material, fitted over said piston engaging portion between said piston rod and said piston head for releasably closing said transverse passage. Said piston assembly is configured such that: when said piston assembly is stationary with respect to said cylinder, said piston remains in its engaging position with said piston engaging portion so as to close said transverse passage and block the fluid communication of said liquid reservoir with the outside of said container through said fluid passage; as said piston assembly is moved downwards relative to said cylinder, said piston deforms to offset from said engaging position so as to open said transverse passage in said piston engaging portion and allow said liquid reservoir to be communicate with the outside of said container through said fluid passage; as said piston assembly is moved upwards relative to said cylinder, said piston remains in said engaging position so as to close said transverse passage and block the fluid communication of said liquid reservoir with the outside of said container through said fluid passage. The piston assembly is featured in that said piston comprises a substantially cylindrical body with opposite open ends, said body, at its upper end, being formed with an annular inner flange radially protruding inwardly for fitting around a periphery of said piston engaging portion so as to releasably close said transverse passage, and, at its lower end, being formed with a sealing lip slidably abutting against a sidewall of said cylinder, said sealing lip being an end edge of a lower portion of said body which is gradually flared and thinned as said lower portion extends downwards, said body, at its upper end where said annular inner flange is formed, being formed with an annular notch along a peripheral edge of said body.

With the above piston assembly of the invention, because the piston made of deformable elastomeric material, at its upper end where the annular inner flange is formed, is circumferentially formed with the annular notch along its peripheral edge, which reduces the rigidity of this portion of the piston, the increment of rigidity and difficulty to deform owing to the transverse inner flange of this portion of the piston is avoided, thereby improving the operation reliability of the piston and thus of the piston assembly. Moreover, the annular notch can be engaged by the inner sleeve of the integrated threaded sleeve-cylinder cap so as to form an annular sealing interface, which, in cooperation with the sealing between the lower sealing lip of the piston and the cylinder sidewall, helps to prevent the liquid in the container from leakage through the balancing vent formed in the cylinder sidewall.

Additional features and advantages will be apparent as the exemplary embodiments of the invention are described below in reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 a is a sectional view of a liquid pump of the prior art;

FIG. 1 b is a sectional view of the liquid pump shown in FIG. 1 a, illustrating its piston assembly being in the course of moving downwards;

FIG. 1 c is a side elevation view of an integrated piston rod-piston engaging portion-piston head used in the liquid pump of FIG. 1 a;

FIG. 1 d is a sectional view taken along the line A-A in FIG. 1 c;

FIG. 1 e is a sectional view of the piston used in the liquid pump of FIG. 1 a;

FIG. 2 a is a sectional view of another liquid pump of the prior art;

FIG. 2 b is a sectional view of the liquid pump shown in FIG. 2 a, illustrating its piston assembly being in the course of moving downwards;

FIG. 2 c is a side elevation view of an integrated piston rod-piston engaging portion-piston head used in the liquid pump of FIG. 2 a;

FIG. 2 d is a sectional view of the integrated piston rod-piston engaging portion-piston head taken along the line B-B in FIG. 2 c;

FIG. 2 e is a sectional view of the piston used in the liquid pump shown in FIG. 2 a;

FIG. 3 a is a sectional view of a liquid pump of an embodiment of the invention, with the press head shown in the upper stop position of its press stroke with respect to the integrated threaded sleeve-cylinder cap;

FIG. 3 b is a sectional view of the liquid pump shown in FIG. 3 a, illustrating its piston assembly being in the course of moving downwards;

FIG. 4 a is a side elevation view of a piston rod used in the liquid pump of FIG. 3 a;

FIG. 4 b is a sectional view of the piston rod taken along the line C-C in FIG. 4 a;

FIG. 4 c is a perspective view of the piston rod of FIG. 4 a;

FIG. 5 a is a top view of the piston head used in the liquid pump of FIG. 3 a;

FIG. 5 b is a sectional view of the piston head taken along the line D-D in FIG. 5 a;

FIG. 5 c is a side elevation view of the piston head of FIG. 5 a;

FIG. 5 d is a perspective view of the piston head of FIG. 5 a;

FIG. 6 a is a side elevation view of the piston used in the liquid pump of FIG. 3 a;

FIG. 6 b is a sectional view of the piston taken along the line E-E in FIG. 6 a;

FIG. 6 c is a perspective view of the piston of FIG. 6 a;

FIG. 7 is a sectional exploded view of the piston assembly used in the liquid pump of FIG. 3 a;

FIG. 8 is a perspective view of the piston assembly of FIG. 7, with part of the piston being cut away to clearly show the configuration of the piston engaging portion;

FIGS. 9-10 are views similar to the views of FIGS. 3-4, showing another embodiment of the piston rod and the piston head of the invention; and

FIGS. 10-11 are views similar to the views of FIGS. 3-4, showing still another embodiment of the piston rod and the piston head of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 3-6, a liquid pump of the invention mainly comprises a piston assembly 10, a press head 20, a cylinder 30, an integrated threaded sleeve-cylinder cap 40, a spring 50 and a lower non-return ball valve 60. The liquid pump can, through its integrated threaded sleeve-cylinder cap 40, be mounted at an opening (usually an open neck) of a container (not shown) of a liquid product. Of course, the threaded sleeve and cylinder cap can also be of the separate type. As the invention focuses on the piston assembly of liquid pump, other components of the liquid pump, which may be of the conventional structures, are not described in detail in this application.

The piston assembly 10 is composed of a piston rod 11, a piston head 12 and a piston 13 fitted around a piston engaging portion 15 between the piston rod 11 and the piston head 12. Both the piston rod 11 and the piston head 12 can be made of a relatively rigid plastic material, for example, polypropylene, and the piston 13 is made of a relatively elastic elastomeric material, for example, NBR and thermoplastic polyurethane. The piston rod 11 is, in its center, formed with a fluid passage 14 having plural sections of length of different diameters. The piston rod 11 is, at its upper end, formed with a press head connecting portion 1101, which is circumferentially formed with annular ribs 1107 on its periphery. The annular ribs are engaged with corresponding annular grooves formed in an inner circumference of a connecting tube 21 of the press head 20, thereby connecting the piston rod 11 to the press head 20. The piston rod 11, at its lower end, is formed with a reduced diameter tube portion 1103, which serves as a piston-engaging-portion formation part to constitute the piston engaging portion 15 which engages the piston 13. A conical step surface 1104 is provided immediately adjoining and tapering towards the reduced diameter tube portion 1103, and is provided by an annular projection formed on the piston rod 11 to provide a space for the piston 13 to deform towards the side of the conical step surface 1104. The reduced diameter tube portion 1103 which constitutes the piston engaging portion 15 can, at its joint with the conical step surface 1104, have a length of increased diameter tube portion 1108 with a lightly increased diameter. A shoulder 1102 is formed on the periphery of the piston rod 11 for engaging a corresponding portion provided on the integrated threaded sleeve-cylinder cap 40 so as to assist defining the upper stop position of press stroke of the press head 20 relative to the integrated threaded sleeve-cylinder cap 40.

The piston head 12 includes a base 121 and a guide stub 123 extending upwards from the base 121. The guide stub 123 can be inserted and secured into a stub receiving hole 1105 that is formed at the lower end of the piston rod 11, which stub receiving hole constitutes part of the fluid passage 14. An annular groove 126 is circumferentially formed in the periphery of the guide stub 123 to be engaged with a corresponding annular rib 1106 formed in the stub receiving hole 1105 of the piston rod 11 so as to enhance fixture of the guide stub 123 in the stub receiving hole 1105. A cylindrical reduced diameter boss 122 with its diameter being smaller than that of the base 121 is formed on the upper side of the base 121, and serves as another piston-engaging-portion formation part 15. The diameter of the reduced diameter boss 122 is greater than that of the guide stub 123, and is the same as that of the reduced diameter tube portion 1103. Therefore, when the piston head 12 is fitted to the piston rod 11, the reduced diameter tube portion 1103 of the piston rod 11 and the reduced diameter boss 122 of the piston head 12 abut against each other to cooperatively constitute the piston engaging portion 15.

Two (or one or any other suitable number) longitudinal slots 124 are symmetrically formed in the outside of the guide stub 123 and constitute longitudinal passages within the stub receiving hole 1105. Corresponding number of transverse passages 125 are formed at corresponding positions in the reduced diameter boss 122, and extend inward from the periphery of the reduced diameter boss 122 so as to communicate with the longitudinal slots 124, whereby the longitudinal slots 124 provide passages from the transverse passages 125 through the longitudinal slots 124 to the fluid passage 14. For easy molding, the transverse passages 125 are integral with the longitudinal slots 124 without any clear boundary therebetween. The base 121 is provided, at its joint with the reduced diameter boss 122, with a steep step surface 127 that is perpendicular to the longitudinal direction and is used to limit deformation of the piston 13 towards the side of the steep step surface 127.

The piston 13 comprises a substantially cylindrical body with opposite open ends. A radially inwardly extending annular flange 131 is formed at an upper end of the body, and forms an inner circumferential surface for fitting around the outer circumferential surface of the piston engaging portion 15 to releasably close off the transverse passages 125. The body is formed at its lower end with a sealing lip 132 for slidably abutting against the sidewall of the cylinder 30. The sealing lip 132 is a lower edge of a lower portion of the body which is gradually flared and thinned as the lower portion extends downwards. An annular notch 133 is circumferentially formed, at an upper end of the body where the annular inner flange 131 is formed, along a peripheral edge of the body. Provision of this annular notch 133 appropriately reduces the rigidity of this portion of the piston 13 and thus appropriately counteracts the increment of rigidity of this portion due to the presence of the annular flange 131, thereby improving the deformability of the piston 13. Of course, a conventional piston without this annular notch 133 (such as those discussed in the Background of the Invention) can also be used in the piston assembly of the invention.

Now the operation of the liquid pump of the invention is described with reference to FIGS. 3 a and 3 b. The liquid pump can be assembled by fitting the upper end 33 of the cylinder 30 over the inner sleeve 42 of the integrated threaded sleeve-cylinder cap 40, which is then mounted on a container of liquid product (not shown). In the case that the liquid pump is in non-operative condition, i.e., its press head 20 is not pressed, as shown in FIG. 3 a, the press head 20 is at the upper stop position of press stroke relative to the integrated threaded sleeve-cylinder cap 40, and the piston assembly 10 is in its uppermost position. In this condition, the shoulder 1102 of the piston rod 11 abuts against the corresponding shoulder formed on the integrated threaded sleeve-cylinder cap 40, and the top surface of the annular notch 133 of the piston 13 is abutted by the lower end of the inner sleeve 42 of the integrated threaded sleeve-cylinder cap 40. In this position, the balancing vent 32 formed in the sidewall of the cylinder 30 is below the abutment point between the piston 13 and the inner sleeve 42 while above the abutment point between the piston 13 and the sidewall of the cylinder 30, that is, the balancing vent is closed between the two abutment points. Therefore, the liquid in the container can be prevented from entering into the cylinder 30 through the balancing vent 32 and then leaking out of the container. The piston 13 maintains properly engaging around the outer circumferential surface of the piston engaging portion 15 of the piston assembly 10, so that the transverse passages 125 in the piston engaging portion 15 is closed, thereby preventing the liquid in the liquid reservoir 31 from flowing out of the container through the transverse passages 125, the longitudinal slots 124 in the guide stub 123 and finally the fluid passage 14.

As the press head 20 is pressed to move downwards against the upward force of the spring 50, as shown in FIG. 3 b, the piston assembly 10 is driven by the press head 20 to move downwards. The sealing lip 132 of the piston 13 slides downwards while abutting the sidewall of the cylinder 30. Owing to the combined effect of the downward driving force exerted on the annular inner flange 131 of the piston 13 by the piston rod 11 and the piston engaging portion 15 and the upward friction force exerted on the sealing lip 132 of the piston 13 by the sidewall of the cylinder 30, as well as owing to the deformation space provided for the piston 13 by the conical step surface 1104 of the piston rod 11, the piston 13 is forced to deform towards the side of the conical step surface 1104, i.e., the side of the piston rod 11. The deformation causes the annular inner flange 131 of the piston 13 to offset from the piston engaging portion 15, so that the transverse passages 125, which are originally closed by the piston 13, is opened, thereby allowing the liquid in the liquid reservoir 31 to be pumped out of the container through the transverse passages 125, the longitudinal slots 124 and the fluid passage 14 under the pumping action of the downward pressing motion of the piston assembly 10. At this time, the lower non-return ball valve 60 maintains closed by the pressure in the cylinder 30 so as to prevent the liquid in the liquid reservoir 31 from reflowing downwards.

Subsequently, as the press head 20 is released from the downward pressing force, the piston assembly 10 is moved upwards relative to the cylinder 30 by the upward force of the spring 50. At this time, because the steep step surface 127 of the piston head 12 does not provide any space for the piston 13 to deform towards the piston head 12, the piston 13 substantially cannot deform to allow the annular inner flange 131 of the piston 13 to offset from the piston engaging portion 15 to open the transverse passages 125. As such, the communication from the liquid reservoir 31 to the outside of the container is blocked up. As the piston assembly 10 moves upwards, a negative pressure occurs in the liquid reservoir 31 of the cylinder 30 and thus makes the lower non-return ball valve 60 open, whereby a volume of the liquid of is drawn into the liquid reservoir 31 from the container through the valve 60, getting ready for being pumped out in next cycle.

Now production of the piston assembly 10 described above with reference to FIGS. 3-6 is described in detail with reference to FIGS. 7 and 8. Firstly, the piston rod 11, piston head 12 and piston 13 are formed separately by molding. Then, fitting the piston 13 over the guide stub 123 of the piston head 12, and making the reduced diameter boss 122 wholly or partially enter into the central circular hole of the annular inner flange 131. Thereafter, inserting and fixing the guide stub 123 of the piston head 12, which has passed through the central circular hole of the piston 13, into the stub receiving hole 1105 of the piston rod 11, whereby the central circular hole of the piston 13 is properly fitted over the outer circumferential surfaces of both the reduced diameter boss 122 of the piston rod 11 and the reduced diameter tube portion 1103 of the piston rod 11. A piston assembly thus assembled is shown in FIG. 8.

The method is described above to produce the piston assembly of the embodiment shown in FIGS. 3-6 (in which the piston engaging portion is provided by both the piston rod and the piston head), wherein the piston is placed over the piston head first and then the piston head is assembled to the piston rod. However, it is understood that it is feasible to place the piston over the piston rod first (as the piston rod has at least the piston-engaging-portion formation part suitable for receiving the piston), and then assemble the piston head to the piston rod. Anyway, as long as the piston rod or the piston head has a portion for receiving the piston, no matter the piston engaging portion is provided by both the piston head and the piston head or only on one of the two, it is feasible to place the piston over the piston rod or the piston head first and then assemble the piston head or the piston rod to the piston rod or the piston head.

One embodiment of the invention is described above with reference to FIGS. 3-8, in which the piston engaging portion 15 is provided by both of the piston rod 11 and the piston head 12. However, according to the invention, a whole piston engaging portion can be provided by only one of the piston rod and the piston head.

Another embodiment of the invention is shown in FIGS. 9 and 10, in which the piston engaging portion is provided by only the piston rod. In this embodiment, the configuration of both the piston rod and the piston head is substantially the same as that of the embodiment of FIGS. 4 and 5, therefore, the like portions are denoted by the like reference numbers but with an additional inverted comma. In comparison to the embodiment shown in FIGS. 4 and 5, the difference only lies in that: in the embodiment of FIGS. 9 and 10, on the base 121′ of the piston head 12′, there is not provided a reduced diameter boss 122, and a full-size reduced diameter tube portion 1103′ is provided at the lower end of the piston rod 11′. As such, the piston engaging portion 15 is made up of the reduced diameter tube portion 1103′ of the piston rod 11′ singly, unlike the case of the embodiment of FIGS. 4-5 where the piston engaging portion is made up of a combination of the reduced diameter boss 122 on the piston head 12 and the reduced diameter tube portion 1103 on the piston rod 11. At the lower end of the reduced diameter tube portion 1103′, there are formed two transverse passages 1110 which are diametrically opposite and extending from the outer circumferential surface to the inner circumferential surface of the reduced diameter tube portion 1103′. The transverse passages 1110 can be made with a width substantially the same as the width of the longitudinal slots 124′ in the piston head 12′. In the condition that the piston head 12′ is assembled together with the piston rod 11′, the lower end of the reduced diameter tube portion 1103′ of the piston rod 11′ abuts against the upper side of the base portion 121′ of the piston head 12′, with the two transverse passages 1110 substantially aligned with the two longitudinal slots 124′ so as to make them communicate with each other. Thus, a piston assembly thus assembled presents substantially the same structure as that shown in FIG. 8.

Still another embodiment of the invention is shown in FIGS. 11 and 12, in which the piston engaging portion is provided only by the piston head. In this embodiment, the configuration of both the piston rod and the piston head is largely the same as that of the embodiment of FIGS. 4 and 5, therefore, the like portions are denoted by the like reference numbers, but with additional double quotation marks. In comparison to the embodiment shown in FIGS. 4 and 5, the difference only lies in that: in the embodiment of FIGS. 11 and 12, at the lower end of the piston rod 11″ there is not provided a reduced diameter tube portion 1103, and a full-size reduced diameter boss 122″ is provided on the base 121″ of the piston head 12″. As such, the piston engaging portion 15 is made up of the reduced diameter boss 122″ provided on the piston head 12″ singly, unlike the case of the embodiment of FIGS. 4-5 where the piston engaging portion is made up of a combination of the reduced diameter boss 122 on the piston head 12 and the reduced diameter tube portion 1103 on the piston rod 11. At the lower portion of the reduced diameter boss 122″ and in the positions substantially aligned with the longitudinal slots 124″, there are formed transverse passages 125″ extending from the outer circumferential surface of the reduced diameter boss 122″ inwardly so as to communicate with the longitudinal slots 124″. The transverse passages 125″ can be made with a width substantially the same as the width of the longitudinal slots 124″ on the piston head 12″. In the condition that the piston head 12″ is assembled together with the piston rod 11″, the upper side of the reduced diameter boss 122″ of the piston head 12″ abuts the lower end of the piston rod 11″, therefore, the reduced diameter boss 122″ is immediately adjacent to the conical step surface 1104″ on the piston rod 11″. An assembled piston assembly thus assembled presents substantially the same structure as that shown in FIG. 8.

In the exemplary embodiments of the invention described above, for

the sake of simplifying description, many aspects of the invention are implemented in combination in one and the same embodiment. However, those skilled in the art can understand that these aspects of the invention can be separately implemented in different embodiments so as to attain their respective objects. For example, in each of the embodiments described herein, the piston assembly, which is manufactured by separately providing the piston rod and the piston head and then assembling them together, in accordance with one aspect of the invention is used with the piston with the annular notch in accordance with another one aspect of the invention, but it is possible to implement the two in separate embodiments so as to realize their respective functions. Of course, those skilled in the art can also understand that the combined use of the technical solutions of these two aspects of the invention may result in even better technical superiority.

In addition, various changes and modifications can be figured out by those skilled in the art without departing from the spirit of the invention and in the scope thereof For example, in the embodiments described herein, the piston rod and the piston head are configured so that a portion of the piston head is inserted and fitted into a portion of the piston rod, but it is also feasible to configure the piston rod and the piston head so that a portion of the piston rod is inserted and fitted into a portion of the piston head. For another example, in the embodiments described herein, the number of each of the longitudinal passages and the transverse passages in the piston head is two, but the number can be one or more than two. For still another example, in the embodiments described herein, the transverse passages in the piston engaging portion are provided by the reduced diameter boss of the piston head, but the transverse passages can also be provided by both the reduced diameter boss and the reduced diameter tube portion, or provided only by the reduced diameter tube portion of the piston rod. Clearly, these changes will not influence the implementation of the invention and the achievement of the invention object. Therefore, the scope of the invention is not limited to the specific embodiments described herein, but is to be rationally defined by the claims. 

1. A liquid pump using an elastomeric piston, for mounting on a liquid container to pump liquid in said container out of said container, said liquid pump comprising a piston assembly which is disposed in a cylinder slidably in a longitudinal direction of the pump, said cylinder defining a liquid reservoir for accommodating liquid therein, said piston assembly comprising: a piston rod, formed with a fluid passage in a center of the piston rod in the longitudinal direction; a piston head, connected to a lower end of said piston rod; a piston engaging portion in the form of a neck between said piston rod and said piston head, said piston head being connected with said piston rod via said piston engaging portion, said piston engaging portion being formed with a transverse passage that diametrically extending through said piston engaging portion and is in fluid communication with said fluid passage so as to provide a passage for the liquid accommodated in said liquid reservoir to be pumped out of said container; and a piston made of an elastomeric material, fitted over said piston engaging portion between said piston rod and said piston head for releasably closing said transverse passage, said piston assembly being configured such that: when said piston assembly is stationary with respect to said cylinder, said piston remains in its engaging position with said piston engaging portion so as to close said transverse passage and block the fluid communication of said liquid reservoir with the outside of said container through said fluid passage; as said piston assembly is moved downwards relative to said cylinder, said piston deforms to offset from said engaging position so as to open said transverse passage in said piston engaging portion and allow said liquid reservoir to be communicate with the outside of said container through said fluid passage; as said piston assembly is moved upwards relative to said cylinder, said piston remains in said engaging position so as to close said transverse passage and block the fluid communication of said liquid reservoir with the outside of said container through said fluid passage, wherein said piston rod and said piston head are formed separately and then assembled together, said piston engaging portion being composed of a piston-engaging-portion formation part of at least one of said piston rod and said piston head.
 2. The liquid pump of claim 1, wherein said piston head comprises a base and a guide stub extending upwards form said base, said piston rod being formed with a guide stub receiving hole at its lower end, said guide stub receiving hole being formed as part of said fluid passage, said guide stub portion being inserted into and fitted to said guide stub receiving hole so as to assemble said piston head onto said piston rod, said guide stub being formed with at least one longitudinal slot along a longitudinal length of said guide stub, said longitudinal slot constituting a longitudinal passage in said guide stub receiving hole, said piston engaging portion being formed with said transverse passage corresponding to said longitudinal slot in number, with said longitudinal slot correspondingly communicating with said transverse passage in said piston engaging portion, thereby providing a passage from said transverse passage to said fluid passage.
 3. The liquid pump of claim 2, wherein said piston engaging portion is provided by both of said piston rod and said piston head, wherein said piston rod, at its lower end, is formed with a cylindrical reduced diameter tube portion which serves as one of said piston-engaging-portion formation parts, said reduced diameter tube portion defining part of said guide stub receiving hole, and said piston head, at an upper end of its base, is formed with a cylindrical reduced diameter boss which serves as the other of said piston-engaging-portion formation parts, said reduced diameter tube portion and said reduced diameter boss having the same diameter, whereby said reduced diameter tube portion and said reduced diameter boss abut against each other to cooperatively constitute said piston engaging portion when said piston head is assembled to said piston rod.
 4. The liquid pump of claim 3, wherein said transverse passage of said piston engaging portion is formed in said reduced diameter boss.
 5. The liquid pump of claim 4, wherein said transverse passage of said piston engaging portion is formed integrally with said longitudinal slot of said guide stub.
 6. The liquid pump of claim 3, wherein said transverse passage of said piston engaging portion is formed in said reduced diameter tube portion.
 7. The liquid pump of claim 3, wherein said transverse passage of said piston engaging portion is formed in both said reduced diameter tube portion and said reduced diameter boss.
 8. The liquid pump of claim 2, wherein said piston engaging portion is provided only by said piston head, wherein said piston head, at an upper end of its base, is formed with a cylindrical reduced diameter boss which serves as said piston-engaging-portion formation part, when said piston head is assembled to said piston rod, said reduced diameter boss abutting against a lower end of said piston rod to cooperatively constitute said piston engaging portion.
 9. The liquid pump of claim 2, wherein said piston engaging portion is provided only by said piston rod, wherein said piston rod, at its lower end, is formed with a cylindrical reduced diameter tube portion which serves as said piston-engaging-portion formation part, said reduced diameter tube portion defining part of said guide stub receiving hole, when said piston head is assembled to said piston rod, said reduced diameter tube portion abutting against an upper end of said base of said piston head to constitute said piston engaging portion.
 10. The liquid pump of claim 2, wherein said guide stub is formed with a recess or protrusion along its periphery, and a mating protrusion or recess is formed an inner circumferential surface of said guide stub receiving hole, assisting to assemble said piston head to said piston rod.
 11. The liquid pump of claim 1, wherein said piston comprises a substantially cylindrical body with opposite open ends, said body being formed with an annular inner flange radially protruding inwardly for fitting around a periphery of said piston engaging portion so as to releasably close said transverse passage, and being formed with a sealing lip slidably abutting against a sidewall of said cylinder.
 12. The liquid pump of claim 11, wherein said annular inner flange is formed at an upper end of said body, and said sealing lip is an end edge of a lower portion of said body which is gradually flared and thinned as said lower portion extends downwards.
 13. The liquid pump of claim 12, wherein an annular notch is formed, at said upper end of said body where said annular inner flange is formed, along a peripheral edge of said body.
 14. The liquid pump of claim 13, wherein said liquid pump further comprises an integrated threaded sleeve-cylinder cap and a press head, said cylinder being connected with said integrated threaded sleeve-cylinder cap on a lower side thereof, said press head being connected to an upper end of said piston rod, said press head having an upper stop position and a lower stop position of its press stroke with respect to said integrated threaded sleeve-cylinder cap, said integrated threaded sleeve-cylinder cap including an inner sleeve and an outer sleeve which extend in the longitudinal direction, said outer sleeve for engaging an open neck of said container so as to mount said liquid pump on said container, said inner sleeve for engaging an upper end of said cylinder so as to connect said cylinder to said integrated threaded sleeve-cylinder cap on the lower side thereof and thus to form a closed cylinder, when said press head is at the upper stop position of said press stroke with respect to said integrated threaded sleeve-cylinder cap, a lower end of said inner sleeve being engaged in said annular notch of said piston.
 15. The liquid pump of claim 14, wherein said cylinder is formed in its sidewall with a balancing vent for balancing pressures within and outside said container, when said press head is at the upper stop position of said press stroke with respect to said integrated threaded sleeve-cylinder cap, said balancing vent being above an abutment point of said sealing lip of said piston with the sidewall of said cylinder, and below the abutment point of said inner sleeve of said integrated threaded sleeve-cylinder cap with said piston.
 16. The liquid pump of claim 1, wherein said piston rod, at its joint with said piston engaging portion, has a conical step surface tapering towards said piston engaging portion, and said piston head, at its joint with said piston engaging portion, has a steep step surface which is substantially perpendicular to the longitudinal direction, said conical step surface providing a space for said piston to deform.
 17. The liquid pump of claim 16, wherein said piston engaging portion, at its joint with said conical step surface, has an increased diameter portion.
 18. A method of manufacturing said piston assembly of said liquid pump of claim 1, said method comprising: separately molding and providing said piston rod, said piston head and said piston; fitting said piston over one of said piston rod and said piston head that is suitable for receiving said piston; assembling said piston rod and said piston head together, with said piston being fitted around said piston engaging portion between said piston rod and said piston head.
 19. A piston assembly for use in a liquid pump for mounting to a liquid container to pump liquid in said container out of said container, said liquid pump comprising a piston assembly which is disposed in a cylinder slidably in a longitudinal direction of the pump, said cylinder defining a liquid reservoir for accommodating liquid therein, said piston assembly comprising: a piston rod, formed with a fluid passage in a center of the piston rod; a piston head, connected to a lower end of said piston rod; a piston engaging portion in the form of a neck between said piston rod and said piston head, said piston head being connected with said piston rod via said piston engaging portion, said piston engaging portion being formed with a transverse passage that diametrically extending through said piston engaging portion and is in fluid communication with said fluid passage so as to provide a passage for the liquid accommodated in said liquid reservoir to be pumped out of said container; and a piston made of an elastomeric material, fitted over said piston engaging portion between said piston rod and said piston head for releasably closing said transverse passage, said piston assembly being configured such that: when said piston assembly is stationary with respect to said cylinder, said piston remains in its engaging position with said piston engaging portion so as to close said transverse passage and block the fluid communication of said liquid reservoir with the outside of said container through said fluid passage; as said piston assembly is moved downwards relative to said cylinder, said piston deforms to offset from said engaging position so as to open said transverse passage in said piston engaging portion and allow said liquid reservoir to be communicate with the outside of said container through said fluid passage; as said piston assembly is moved upwards relative to said cylinder, said piston remains in said engaging position so as to close said transverse passage and block the fluid communication of said liquid reservoir with the outside of said container through said fluid passage, wherein said piston comprises a substantially cylindrical body with opposite open ends, said body, at its upper end, being formed with an annular inner flange radially protruding inwardly for fitting around a periphery of said piston engaging portion so as to releasably close said transverse passage, and, at its lower end, being formed with a sealing lip slidably abutting against a sidewall of said cylinder, said sealing lip being an end edge of a lower portion of said body which is gradually flared and thinned as said lower portion extends downwards, said body, at its upper end where said annular inner flange is formed, being formed with an annular notch along a peripheral edge of said body.
 20. The piston assembly for use in a liquid pump of claim 19, wherein a longitudinal thickness of said annular inner flange is greater than a radial thickness of said body of said piston. 